1. Technical Field
The present invention relates to a Moire clear circuit, and more particularly, relates to a Moire clear circuit for visually canceling Moire interference phenomena on color cathode ray tube "CRT" display monitors for multimode operations.
2. Related Art
Generally, a display device such as a CRT type of monitor is a peripheral device which processes information data received from an information data system such as a computer system and provides a visual display of processed information data on a screen. Such a monitor is typically available in either black and white known as monochrome monitor or high resolution color known as color graphics adapter (CGA), video graphics adapter (VGA) and enhanced graphics adapter (EGA) monitor. In all display devices, the video signal is typically received from a video card installed in an information data system in accordance with vertical and horizontal synchronization signals for a visual display.
Conventional color display device generally includes a micro-processor which receives the vertical and horizontal synchronization signals from the video card to control the visual display of information data on a screen, and vertical and horizontal deflection circuits which respectively receive the vertical and horizontal synchronization signals to perform vertical and horizontal deflection, such that an electron beam generated from an electron gun of a CRT is deflected in a regular sequence from an upper left portion to a lower right portion of a phosphor deposited CRT screen by way of a deflection yoke in order to form an image. The shadow mask has a multiple of minute apertures or slots in a predetermined regular pattern related systematically to the pattern of triads of the luminescent phosphor dots on the phosphor deposited screen, and allows the electron beam corresponding to three primary colors, for example, red, green and blue generated from an electron gun configured of three electrodes for the three primary colors to selectively strike its respective phosphor dot.
Since the electron beam generated from the electron gun forms substantially equally spaced scanning lines when sweeping across the phosphor deposited screen, shades delimited by the regularly patterned minute apertures or slots in the perforated shadow mask and the similarly regularly spaced scanning lines interfere with each other resulting in the appearance of undesirable Moire patterns on the screen of the color display device.
From an operating standpoint, the Moire interference phenomenon poses a serious aesthetic problem, since the best electron beam focus and highest image resolution results in unacceptable noticeable Moire patterns. One traditional solution to the Moire interference problem is to reduce the apertures of the shadow mask and phosphor dot pitch which raise the effective spatial frequency of the CRT, thereby raising the Moire beat frequency so that it is less visible. This solution, however, results in a lower resolution image displayed on a CRT which is inherently capable of significantly higher resolution. Another known solution is to increase a spot size of the electron beam, which also results in a lower resolution image displayed on the CRT and is harmful to the eye sight of the user. Recent solution to the Moire interference problem is to shift the phase of a video sync signal such that the phase of each video scan line is shifted relative to the phase of each immediately preceding video scan line in order to cancel the Moire pattern which occurs on the same video scan line without sacrificing image resolution or brightness of the displayed image. Contemporary Moire cancellation techniques which incorporate different solutions to the Moire interference problem are disclosed, for example, in U.S. Pat. No. 4,887,010 for Display Device Using Shadow Mask CRT issued to Fujimura, U.S. Pat. No. 4,983,879 for Shadow Mask Type Color Cathode Ray Tube With Shadow Mask Effective To Minimize The Appearance Of Moire Patterns issued to Kawaguchi, U.S. Pat. No. 5,107,188 for Method And Apparatus For Cancellation Of Moire Interference In Color Cathode Ray Tube Displays issued to Rindall, U.S. Pat. No. 5,241,237 for Electron Gun And Cathode-Ray Tube issued to Misono et al., U.S. Pat. No. 5,430,502 for Apparatus For Eliminating Moire Pattern Effects Resulting From The Use Of Different Display Resolution With A Fixed Size Shadow Mask issued to Yamazaki et al., U.S. Pat. No. 5,440,353 for Display Monitor Using Moire Cancellation Circuit issued to Yamazaki et al., U.S. Pat. No. 5,473,223 for Vertical Deflection Waveform Generating Apparatus issued to Murakami, U.S. Pat. No. 5,619,094 for Color Cathode Ray Tube And Display Device With Reduced Moire issued to Vriens, and U.S. Pat. No. 5,635,803 for Display Device With Shadow Mask CRT issued to Fujimura et al.
For recent Moire cancellation technique which incorporates a Moire clear circuit for shifting the phase of a video sync signal such as disclosed in U.S. Pat. No. 5,107,188 issued to Rindall and U.S. Pat. No. 5,440,353, as I have observed however, is that internal circuitry is complex and cost prohibitive. Moreover, such a complexity often results in difficulty in vertical synchronization for operation of overall system. Accordingly, it is my observation that further improvement in Moire clear circuit construction can be contemplated.